Elevated plasma levels of apolipoprotein B (apo B) and low density lipoprotein (LDL) are associated with a higher risk for atherosclerotic coronary heart disease (CHD), a leading cause of mortality in the industrialized world. Elevated plasma apo B levels and overproduction of apo B are also major characteristics of familial combined hyperlipidemia (FCHL), a prevalent disorder in the general population. The genetic basis of this prevalent disease is heterogeneous and unknown. Genes that regulate plasma apo B levels are logical candidates for genes which contribute to the FCHL phenotype. Apo B is required for the secretion of very low density lipoproteins (VLDL) from the liver and is the mandatory protein constituent of both plasma VLDL and LDL. Plasma apo B levels are controlled, in part, by secretion rates of apo B-containing lipoprotein particles. Using the human apo B transgenic mouse model (HuBTg), we have shown that HuBTg mice of C57BL/6 (B6) and 129/Sv (129) background differ in their plasma apo B levels mainly due to hepatic apo B secretion rates via post-transcriptional regulation. Further genetic studies of two mouse strains identified two novel quantitative trait loci (QTL) on chromosomes 6 and 4 which have major effects on plasma apo B levels. These genes are designated Apo B regulatory genes (Abrg). Our fine-mapping analyses have localized the Abrgl to an interval of approximately 2.9 Mb on chromosome 6 and the Abrg2 to a 12-cM interval on chromosome 4. The goals of the current proposal are to identify and characterize the Abrgl gene on chromosome 6. The specific aims are: 1. Positional cloning of the Abrgl on chromosome 6 that regulates plasma apo B levels. Several parallel approaches will be taken to identify candidate genes for the Abrgl. These approaches include RT-PCR, quantitative real-time PCR assays, and DNA sequencing analysis. Studies will also be designed to determine the effects of allelic variations of expression variants on transcription. 2. Characterization of the Abrg genes in congenic mice and functional studies of the Abrgl candidate genes. We will perform functional tests on candidate variant genes, including the Copsla gene (an expression variant), by over expression studies in vitro and in vivo. The best candidate for the Abrgl gene, either Cops7a or another gene to be identified, will then be verified via knock-in and/or knock-out gene-targeting techniques. Studies will be designed to characterize the functional roles of the Abrgl gene. Overall, the gene identified will be a novel regulator affecting the pathways involved in the assembly and secretion of apo B containing lipoproteins. As a regulator of apo B secretion rates, this novel gene will be a strong candidate for one of the genes causative for overproduction of apo B in subsets of FCHL patients. It will also be a potential target for drug discovery and pharmacological intervention in hyperlipidemic patients in the general population.